Virtual Local Area Networks (VLANs) have long been used as both an identifier of a service/traffic as well as a Broadcast Domain for flooding in Ethernet networks. Generally, VLAN identifiers (VIDs) have been limited to 4K (4096) unique numbers, although due to the increased demands of current networks, recent standards (e.g., IEEE 802.1ah) provides for greater scalability of service-identifiers (and MAC-tables,) while continuing to use traditional bridging techniques. For instance, one way the scalability problem has been addressed is by identifying a service using stacked VLAN tags and hence providing a 24-bit (12+12) identifier, or up to 16M unique numbers. The bridging of traffic in this scenario is thus performed by mapping the service identifier to a “bridge-domain” object on a given bridge, where MAC learning is performed on this “bridge-domain” and the VLAN tags are simply treated as encapsulation information. This scheme of using VLAN tags as a service identifier is contrary to the conventional meaning of a VLAN as a broadcast-domain in the layer-2 (L2) network.
In addition, redundancy links may be used for high availability reasons, and the redundant links generally benefit from loop-prevention protocols. The Multiple Spanning Tree (MST) Protocol (or MSTP) is widely used in L2 networks for loop prevention, however, MST protocol today assumes that VLANs are used as a service identifier with a network wide scope. That is, MSTP assumes that VLAN encapsulation is the same throughout the entire network, where service identifiers are typically a trunk-based identification. As such, MSTP does not directly apply to an L2 network with trunks carrying more than 4K services using double-stacked VLAN tags (or other encapsulation fields).